Study to Treat Relapsed Follicular Non-Hodgkin's Lymphoma With Radiation and Bexxar

• The Primary Endpoint of the Study Will be to Determine the Feasibility of Combining External Beam Radiotherapy (EBRT) and Bexxar by Assessing the Toxicities Associated With the Treatment. [ Time Frame: 2 yr 3 mos ] [ Designated as safety issue: Yes ]
  13 patients will be enrolled initially and followed for 3 months. If less than 10 of these patients reach a grade III or IV toxicity, then 12 more patients will be enrolled and the study will be deemed feasible. If 11 or more of the first group experience grade III/IV toxicity, the trial will stop early.
  
  

• The Secondary Endpoint Will be to Assess Response Rates and Patterns of Failure in Patients Treated With Bexxar and External Beam Radiotherapy (EBRT). [ Time Frame: 2 yr 3 mos ] [ Designated as safety issue: No ]
  Tumor response to treatment is measured using the RECIST criteria: Response Evaluation Criteria in Solid Tumors which defines Complete Response, Partial Response, Progressive Disease, and Stable Disease, by using tumor measurements as seen on CT or MRI
  
  

Drug: Iodine I -01 Tositumomab (Bexxar)
Patients will be treated in two dosing phases, each phase includes two infusions. The first phase, termed "dosimetric dose," involves an intravenous administration of 450 mg unlabeled tositumomab followed by an intravenous administration of 5 mCi (0.18 GBq) of I-131 tositumomab for the purpose of determining the rate of whole body clearance of radioactivity (residence time) so that the administered activity (mCi or GBq) to deliver a 75 cGy (or 65 cGy for patients with baseline platelet count from 100,000 to 149,999/mm^3) total body radiation dose can be calculated.
Procedure: External Beam Radiation Therapy
All patients are to receive 20 Gy in 10 fractions of 200 cGy to the PTV

Total dose delivered and tumor size are important predictors of local control in the treatment of low-grade Non-Hodgkin's Lymphoma (NHL). The basic principle is that larger nodal masses require increased doses of External Beam Radiotherapy (EBRT) to achieve local control. Radioimmunotherapy (RIT) seems to share this same characteristic. Review of the published literature on both Bexxar and Zevalin reveals that one of the most important predictors of treatment failure is nodal volume and its apparent relationship to dose delivered by RIT. The best tumor dosimetry for RIT is from Dr. Wiseman et al reporting on the dosimetry of Zevalin (PMID:11418315). He showed that tumors ≥15 cm^3 received only 1082 cGy with Zevalin, whereas the average dose delivered in tumors <15 cm^3 was 4763 cGy. Recently, Gokhale et al (PMID:16111589) published their experience with Zevalin at Cleveland Clinic and showed a significant correlation with pretreatment tumor volume and response to therapy. In their experience, tumors ≥5 cm had an 83% rate of local recurrence versus 28% for tumors <5 cm. This dosing paradox (bigger masses, which require more dose, receive less with RIT) may be diminished by the delivery of additional EBRT. This is the hypothesis that underlies the pilot study.

The dosimetric data available for Bexxar is more heterogeneous but confirms the observations seen with Zevalin. In patients previously untreated for low-grade Non-Hodgkin's Lymphoma (NHL), Koral et al (PMID:12621015) showed an increased likelihood of achieving a complete response (CR) if tumor doses were >650 cGy. Previous work by these same authors showed a trend for larger tumor volumes receiving less dose (PMID:10994741). The most compelling data for this relationship comes from the clinical trials done using Bexxar. Both in the pivotal trial (PMID:11579112) and the recently published trial treating naïve patients (PMID:15689582), tumor volume was a significant predictor of response to Bexxar. In the pivotal trial, smaller tumor burden was the only factor predicting longer duration of response.

Whereas EBRT might be able to provide reliable radiation dose, the use of Bexxar may provide the therapeutic equivalent of central lymphatic irradiation, which would permit the use of true involved field radiotherapy. Investigators have previously noted that increased EBRT field size is associated with increased short-term and long-term toxicity. The toxicities associated with the treatment of radiotherapy are related to the site treated, but do not necessarily include the dose limiting toxicity of Bexxar, which is primarily hematologic and transient. As the toxicity of RT and Bexxar may not overlap, the combination of both may allow an increase in the therapeutic window for both radiotherapy and Bexxar therapy.